<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // 事件触发器
    class EventEmitter {
      constructor () {
        // {click: [fn1, fn2], click2: [fn]}
        this.subs = Object.create(null)
      }

      // 注册事件
      $on (eventType, handler) {
        this.subs[eventType] = this.subs[eventType] || []
        this.subs[eventType].push(handler)
      }

      // 触发事件
      $emit (eventType) {
        if ( this.subs[eventType] ) {
          this.subs[eventType].forEach(handler => 
            handler()          
          )
        }
      }
    }

    // 测试
    let em = new EventEmitter()
    em.$on('click', () => {
      console.log('click')
    })
    em.$on('click', () => {
      console.log('click2')
    })
    em.$emit('click')
  </script>
</body>
</html>